The present invention relates to an active control-type control apparatus of a magnetic bearing.
A magnetic bearing is superior in many points, for example that it can be rotated at ultrahigh speeds without lubrication being taken since a rotary body of the magnetic bearing is supported without contact in space by the magnetic force thereof. However, when the rotary shaft is rotated at ultrahigh speeds, a control system of the magnetic bearing disadvantageously becomes unstable as a result of the gyro-effect.
Conventionally, a cross feedback method has been employed in the control system of the magnetic bearing having two degrees of freedom for the rotational motion so as to eliminate the above-described phenomenon, that is, the unstable state of the control system during rotation at ultrahigh speeds resulting from the gyro-effect.
The structure of a conventional magnetic bearing control apparatus of the type referred to above is indicated in FIGS. 4 and 5.
More specifically, the control apparatus which controls three degrees of freedom for the translation motion (x, y, z) of the center of gravity G of a rotary shaft 1 and two degrees of freedom for the rotational motion (.theta..sub.X, .theta..sub.Y) around the center of gravity G are provided with displacement sensors 2 and 3 for measuring a displacement in an x-axis direction, displacement sensors 4 and 5 for measuring a displacement in a y-axis direction, and a displacement sensor 6 for measuring a displacement in a z-axis direction. The rotating state of the rotary shaft 1 is detected by these sensors. Moreover, electromagnets 7 through 14 are provided to impress a force to the rotary shaft 1, and electromagnets 15 and 16 are mounted to impress a force in the z-axis direction.
The rotational motion of the rotary shaft 1 measured by the displacement sensors is fed back to the five control devices, namely, the control devices of three degrees of freedom for the translation motion and two degrees of freedom for the rotational motion. Each of the five control devices outputs a current instructing signal for the electromagnets to respective amplifiers A-J based on the feedback signal so as to keep the rotary shaft 1 floating in a predetermined state. The amplifiers A-J then amplify the current instructing signal, feeding a current to the electromagnets 7-16, thus impressing a force to the rotary shaft 1.
The magnetic bearing control apparatus is generally formed in the above-described fashion. Now, the .theta..sub.Y -axis control device and .theta..sub.X -axis control device of FIG. 4 will be more specifically described for explanation of the conventional compensation of the gyro-effect, i.e., the cross feedback method.
FIG. 5 is a block diagram of the two control devices respectively for controlling the .theta..sub.Y -axis and .theta..sub.X -axis rotational motion. A rotational displacement .theta..sub.Y around the center of gravity of the rotary shaft 1 is operated by a .theta..sub.Y operating circuit 17 from output signals of the x-axis displacement sensors 2 and 3, and output to a deviation circuit 18. The deviation circuit 18 obtains the difference between a .theta..sub.Y -axis position reference signal generated from a .theta..sub.Y -axis position reference meter 19 and the rotational displacement .theta..sub.Y, outputs a deviation signal to a compensation circuit 20. After processing the deviation signal by P.I.D. compensation or phase compensation, the compensation circuit 20 outputs a control signal to an adder 21.
Likewise, in the .theta..sub.X -axis control device, a rotational displacement .theta..sub.X around the center of gravity of the rotary shaft 1 is operated by a .theta..sub.X -axis operating circuit 22 from output signals of the y-axis displacement sensors 4 and 5, and outputs a signal to a deviation circuit 23. The deviation circuit 23 obtains the difference between a .theta..sub.X -axis position reference signal from a .theta..sub.X -axis position reference meter 24 and the rotational displacement .theta..sub.X and then outputs a deviation signal to a compensation circuit 25. In the compensation circuit 25, the deviation signal is subjected to P.I.D. compensation or phase compensation, and the resultant control signal is output to an adder 26.
The two degrees of freedom in the rotational motion are stabilized in the foregoing manner of control. However, the magnetic bearing becomes unstable when the rotary shaft 1 is rotated at ultrahigh speeds. The cross feedback method mentioned earlier is accordingly carried out to eliminate the unstable state between the two degrees of freedom in the rotational motion. In other words, the adder 21 adds the .theta..sub.Y control signal to a signal obtained by multiplying the .theta..sub.X rotational displacement by a gain (-G.sub.2), thereby generating a current instruction i.theta..sub.Y. At the same time, the adder 26 adds the .theta..sub.X control signal to a signal obtained by multiplying the .theta..sub.Y rotational displacement by a gain (G.sub.1), thereby outputting a current instruction i.theta..sub.X. The effect of this cross feedback method will be described with reference to a diagram of the gain of the rotational motion control system.
In the diagram of FIG. 6, the gain of the rotational motion control system is compared between when the cross feedback method is used (shown by an alternate long and short dashed line) and when the cross feedback method is not used (shown by a dotted line). A frequency response shown by a solid line represents a closed-loop transfer function of the rotational motion control system when the rotational speed .omega. of the rotary shaft 1 is equal to 0. A peak value at this t is 10dB (f=200 Hz). When the rotary shaft 1 is rotated at .omega.=50000 rpm, the closed-loop transfer function is turned as indicated by the alternate long and short dashed line in FIG. 6, with a peak value of 20dB (f=85 Hz). The control system becomes unstable. Meanwhile, if the conventional cross feedback method is tried in the manner as indicated in FIG. 5, such a frequency response as drawn by the dotted line is obtained, whereby the above peak value 20dB (f=85 Hz) is improved.
In the above-described structure, when the frequency is relatively high, for example, 400 Hz in FIG. 6, the peak value is 15dB, which that the control efficiency has deteriorated.